A projection type projector apparatus is required to uniformly project an image with sufficient color rendering properties on a rectangle screen, and a high pressure discharge lamp of high mercury vapor pressure is used as a light source of such a projector apparatus. This is because light having a visible wavelength band can be obtained on a high output by increasing the pressure of mercury vapor.
The extra high pressure mercury lamps which have such a feature are roughly classified into an AC discharge lamp and a DC discharge lamp in terms of the lighting system.
Each of such lamps has its own features, and in the AC discharge lamp, projections are formed at the tips of electrodes, and consumption and growth of almost equal luminescent spots are repeated at a front face of each projection, so that a high lumen maintenance factor can be obtained for a long time. The high-pressure discharge lamp lighting apparatus using such an AC discharge lamp is disclosed, for example, in patent documents 1. In patent document 1, in order to stably maintain the distance between electrodes, when the lighting voltage of the discharge lamp drops less than a lower limit, the lighting frequency of the discharge lamp is decreased so that the lighting voltage is increased.
On the other hand, since in a DC discharge lamp, only one luminescent spot is generated in front of a cathode thereof, it is possible to use light efficiently in a small optical device. Although this technology is advantageous in the miniaturization of recent projectors, since the anode side is worn out constantly, it is difficult to keep the arc length constant, and a lumen maintenance factor is not good. By the way, types of projector apparatuses are in general divided into a system using DLP (registered trademark) and a system using a liquid crystal panel.
In the system using the DLP (registered trademark), as shown in FIG. 10, radiation light emitted from a light source 110 is irradiated in a time dividing manner on a special modulation element 115 (which is also called a light modulation device, and is, for example, a DMD element (Digital Micro-mirror Device)) through a lens 111, a color wheel (rotation filer) 112 in which divided RGB areas are formed, a rod integrator 113, and a lens 114, and specific light is reflected by the DMD element so as to be irradiated onto a screen 117 through a lens 116 for projection. The DMD element is formed by arranging a couple of million of tiny mirrors, each of which corresponds to a pixel, and the direction of each tiny mirror is controlled so as to control light projection. Since it is not necessary to use three liquid crystal panels while the DLP (registered trademark) system has a simple optical system, as compared with a liquid crystal system, it is advantageous that the structure of the entire apparatus can be miniaturized and simplified.
On the other hand, in the liquid crystal panel, there are an one-sheet type liquid crystal panel and a three-sheet type liquid crystal panel. In either type, light emitted from a light source is separated into three colors (RGB), and light transmission adjustment is carried out on light which is related to image information in a liquid crystal panel. After that, the three colors which transmitted through the panel, are synthesized so as to project the synthesized image on a screen.
FIG. 11 is a schematic view of the structure of a projector apparatus which has a liquid crystal panel.
As shown in the figure, light emitted from a lamp 100 is made into a uniform light by an integrator lens (fly eye lens) 101, and made into the light with only S polarization by a polarization conversion element 102. This light with S polarization enters to dichroic mirrors 104-106 through a mirror 103, and the white light is separated into RGB. The separated lights are reflected by mirrors 107, after passing an LCD panels 108, it is synthesized by a dichroic prism 109, so that an image is generated.